In a work machine, such as a hydraulic excavator, an oil-hydraulic motor can swing a revolving upper-structure on a base carrier. Since a work machine has a high moment of inertia, the hydraulic pressure in an oil-hydraulic circuit becomes extremely high and causes the relief losses of hydraulic oil while the oil-hydraulic motor is starting and accelerating. A variety of techniques have been proposed for reducing such relief losses.
For example, Patent Literature 1 discloses a technique which decreases the discharge flow rate of a hydraulic pump to reduce relief losses during operation of a swing motor. This technique involves detection of a pilot pressure from a pilot valve linked to a swing lever, detection of a hydraulic pressure over the circuit between a flow rate control valve and the swing motor, and control of a swash plate angle of a hydraulic pump based on these values. Such a configuration can reduce relief losses, and prevent the degradation of the swing motor caused by heat generation and high temperature.